Mata moved to Old Trafford in January after falling out of Jose Mourinho's first-team plans at Stamford Bridge.
The Spain international was a crucial first-team player during Benitez's short reign as Blues boss last season, winning Chelsea's Player of the Year award for the second year running, and the manager says his importance is obvious.
"I am not here to talk about Chelsea," he told reporters. "But Juan was their best player in the last two years. That is it. You can see that. But I am not here to analyze what they did."
Benitez was speaking ahead of Napoli's Europa League visit to Swansea City on Thursday, and he hopes to have Gonzalo Higuain available for selection after the striker picked up a back injury at the weekend.
"I will see how he moves on the field, as I have one more day to evaluate him," he added. "I am optimistic, but will talk it over with him and see. Raul Albiol is not 100 percent either, so we’ll see [on Thursday] how he’s feeling."
Benitez is wary of the Swans side despite recent turmoil at the Liberty Stadium following the sacking of Michael Laudrup.
"Swansea are a different side to most in the Premier League, as they play the ball and have the pace and quality of their Spanish players," he said.
"It's true they changed their manager, but it's never easy to take on a Premier League side. I don't know whether Swansea will focus more on domestic challenges or the Europa League, but we do have the slight advantage of knowing their style of football."
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